A switching DC/DC converter is very popular as a power supply for an electronic device or any other device. A known type of switching DC/DC converter can change or regulate an output voltage in accordance with a setting signal (command) from the control unit of an electronic device or any other external device.
FIG. 10 is a diagram showing the typical circuit arrangement of such a DC/DC converter. The illustrated circuit is a PWM-controlled DC/DC converter. A power supply control IC 7 includes a PWM comparator 53 incorporated in a power supply control IC 7 and having a function of setting the maximum ON-duty of a switching element Q101.
This function is generally called dead-time control. The maximum ON-duty is limited by a control voltage Vdtc of a dead-time control circuit 4 arranged outside the control IC 7. The dead-time control circuit 4 includes a capacitor C1 which also serves as a soft start function of preventing the flow of a rush current at the start of the DC/DC converter. When the power supply is activated, a gradual increase in charge voltage of the capacitor C1 is used to gradually increase the ON duty of the PWM-controlled DC/DC converter. Upon activation, when the converter reaches a steady state (the stable state in which the voltage has reached to the desired voltage), the dead-time control voltage Vdtc becomes a constant value. This voltage value defines the maximum ON duty.
To change an output voltage Vo from the DC/DC converter in accordance with a setting signal from the control unit 3 of the electronic device or an external device, a voltage setting signal from the control unit 3 is input to a DA converter 6. The DA converter 6 converts the input signal into an analog voltage. The analog voltage is input to the V− terminal of an error amplifier 54 which detects the output voltage. The V− terminal voltage is a reference voltage for setting the output voltage Vo. This reference voltage is compared with the V+ terminal voltage of the error amplifier 54 which is obtained by causing resistors R101 and R102 to divide the output voltage Vo. PWM control is performed such that the V+ terminal voltage becomes equal to the V− terminal voltage, thereby adjusting and controlling the ON/OFF duty of the switching element Q101.
The output voltage Vo from the DC/DC converter is given by:Vo=V−×(R101+R102)/R102where V− is the output from the DA converter 6. That is, a voltage proportional to the output from the DA converter can be supplied from the DC/DC converter in FIG. 10 to a load 2 of the electronic device.
As described above, in the DC/DC converter capable of changing the output voltage from the setting signal from the control unit of the electronic device or the external device, the power supply IC 7 and its peripheral circuits have different operations depending on a timing at which the output voltage setting signal from the control unit 3 is output.
A case in which a voltage setting signal is output from the control unit 3 before activating the DC/DC converter, and the reference voltage V− for setting the output voltage Vo from the DA converter 6 is determined will be taken as an example.
When a voltage required for activating the power supply control IC 7 is applied to the input terminal of the DC/DC converter, the power supply system in the control IC is activated, and a reference voltage source 51 is also activated to output a reference voltage Vref. At this time, the output voltage is 0 [V], and the V+ terminal voltage of the error amplifier 54 which detects the output voltage is also 0 [V]. The output voltage from the DA converter 6 which is based on the voltage setting data from the control unit 3 is supplied to the V− terminal voltage of the error amplifier 54. An output VFB from the error amplifier saturates to a minimum voltage (0 [V]). The output VFB from the error amplifier is input to the PWM comparator 53. Reference numeral 8 denotes a circuit which adjusts the phase for stabilizing the feedback control of the error amplifier 54.
In the dead-time control circuit 4, since the reference voltage Vref is 0 [V] before power-on, the voltage between the terminals of the capacitor C1 is also 0 [V]. Upon power-on, when the reference voltage Vref becomes the defined reference voltage, e.g., 2.5 [V], the voltage at the capacitor C1 becomes 0 [V]. Hence, the output Vdtc from the dead-time control circuit 4 becomes 2.5 [V] immediately upon power-on. However, the output voltage Vdtc is gradually discharged via the resistor R3, and exponentially reduced.
FIG. 11 is a graph showing an example of the change of the dead-time control voltage Vdtc upon power-on (upon activation), where Vref=2.5 [V], R2=180 [kΩ], R3=220 [kΩ], and C1=0.47 [μF].
The output VFB from the error amplifier 54 and the output Vdtc from the dead-time control circuit 4 are input to the + input terminal of the PWM comparator 53. A triangular wave having a constant frequency is input from a triangular oscillator 52 to the − input terminal. The PWM comparator 53 compares a triangular wave level and a higher signal level at two + input terminals. The ON/OFF signal VG of the switching element is output via an output driver 55 such that if the triangular wave level is higher, the switching element Q101 is turned on, and if the triangular wave level is lower, the switching element Q101 is turned off.
Therefore, upon power-on, the output VFB from the error amplifier saturates to a minimum voltage (0 [V]). However, since the dead-time control voltage Vdtc is gradually reduced from 2.5 V as shown in FIG. 11, the output from the PWM comparator gradually increases from 0% of the ON duty to prevent the excessive input current flow. In an example shown in FIG. 11, according to the triangular wave level, a peak voltage VTH is 1.97 [V], and the bottom voltage VTL is 1.48 [V]. Such operation upon power-on is called a soft start function of suppressing the input rush current, as in the DC/DC converter which converts the output voltage into a fixed value.
As a similar soft start function, Japanese Patent Laid-Open No. 05-83933 discloses a means for monitoring the input voltage, and resetting the dead-time control voltage when the voltage is abnormally reduced, in order to prevent the excessive current input generated immediately upon restoring a normal input voltage value, when the input voltage is abnormally reduced.
Japanese Patent Laid-Open No. 05-161345 discloses a scheme for monitoring a load current in order to prevent an output voltage from an overvoltage state immediately upon restoring a steady output current value from the overcurrent protective operation state set upon generation of an output overcurrent, and resetting a dead-time control voltage upon generation of an overcurrent.
The operation of the conventional DC/DC converter capable of changing the output voltage is ensured when the setting voltage is determined by the control unit 3 in advance. For example, the conventional DC/DC converter is used as a DC/DC converter having the function of regulating the output voltage in order to adjust variations in characteristics of the load device connected to the DC/DC converter.
Some electronic device requires a DC/DC converter having a function of changing the output voltage while supplying power to the load device. Examples of such a device are an electronic device which must change the power supply voltage upon a change in temperature of the load since the load temperature changes during operation, and an electronic device which must change the power supply voltage within a short period of time because the power supplied from the power supply must be abruptly changed during operation.
The DC/DC converter which applies the power supply voltage to such an electronic device must receive an output voltage change setting signal from the control unit during supply of a given output voltage and must change the output voltage to the setting voltage within the short period of time. This operation will be described using a conventional circuit in FIG. 10.
During operation at a given output voltage Vo(0), the voltage V+(0) obtained by causing the resistors R101 and R102 to divide the output voltage Vo(0) is applied to the + terminal of the error amplifier 54 and becomes equal to the voltage V−(0) applied from the DA converter 6 to the − terminal.
In this case, when the setting signal (command to increase the output voltage) which changes the output voltage is output from the control unit 3, the voltage from the DA converter 6 changes to V−(1) (V−(1)>V−(0)). At this time, the output voltage does not change, and the + input terminal voltage from the error amplifier 54 is still V+(0). Then, a difference occurs between the voltages input to the error amplifier 54. The output VFB is reduced to the minimum voltage 0 [V], and input to the PWM comparator 53. After the sufficient period of time upon power-on, the output voltage from the dead-time control circuit 4 is the Vdtc(∞) obtained by causing the resistors R2 and R3 to divide the Vref voltage. In the example shown in FIG. 11, the output voltage from the dead-time control circuit 4 is lower than the minimum voltage of the triangular wave.
The two + input terminal voltages from the PWM comparator 53 are equal to or lower than the minimum voltage of the triangular wave. The PWM signal output from the PWM comparator 53 has an ON duty of 100%. Hence, the main switching element Q101 is completely turned on. The excessive current flows, and the main switching element Q101 may fail. Furthermore, an operation error may occur in the overcurrent protective circuit for an input-side AC adapter and battery.
Alternatively, when a signal which reduces the setting voltage is output from the control unit 3 in an output state with no load, the output capacitor C102 of the DC/DC converter cannot be discharged, and it takes a long time until the output voltage is reduced to the setting value, thus posing a problem.